Calculating Ejection Charges...

Discussion in 'Mid Power Rocketry (MPR)' started by bronicabill, Mar 24, 2020 at 7:43 PM.

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  1. Mar 24, 2020 at 7:43 PM #1

    bronicabill

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    Somewhere I recently read about how to calculate BP ejection charges based upon the body tube diameter and length. Not sure if I read it here or elsewhere, but I need to do that soon and would like to find the formula to determine a baseline starting point before ground testing in my backyard.

    If someone could point the way, I'd greatly appreciate it! Thanks!!!
     
  2. Mar 24, 2020 at 8:29 PM #2
  3. Mar 24, 2020 at 8:54 PM #3

    bronicabill

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    Thanks! I feel like an idiot for not trying Google first... I know better! Cheers!
     
  4. Mar 25, 2020 at 4:18 AM #4

    Nytrunner

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    I use the rockethead calculator, and the guy who wrote the charge sizing spreadsheet is a club member with us (nasa guy, likes to play with hypergols, use to be club president)
     
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  5. Mar 25, 2020 at 6:58 AM #5

    manixFan

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    Shoot, I was going to link to one that had you enter the mass of the nosecone and the rest of the rocket, and it would calculate velocity of the parts and a bunch of other info, but now I can't find it and it didn't seem to be in the list of search results returned by Google. If anyone has a link to it, please post it!

    Thanks,


    Tony
     
  6. Mar 25, 2020 at 1:43 PM #6

    ebruce1361

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    I'm still not to the point where I need to calculate charges yet, but I'm getting there. So, to deepen my understanding of this, how does one select a desired force to exert on the nose to begin with? Is it based onthe mass of the nose and any associated payload? The force needed to break any shear pins?
     
  7. Mar 25, 2020 at 2:44 PM #7

    Nytrunner

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    Yes.

    For pinned rockets, each 2-56 nylon screw will take ~25-30 lbs to break. Each 4-40 nylons screw will take ~40-45 lbs.

    You will use a certain number and size of pins based on what you need to pin together.
    -Nose/forward section weight times the g's of acceleration upon ejection (some say 50, some say 60)
    -Drag separation force (harder to calculate) [my flights are not extreme, so I don't pin the apogee break, I just make sure the booster doesn't slip off when I pick up the nose/payload]

    Depending on the number of pins you have, you need to generate at least that much force to break the pins, plus some to kick the nose off.
    The pressure needed to generate that force depends on the diameter of the airframe/bulkhead. (10 psi puts more force on a 5.5" bulkhead than a 2.6" bulkhead)

    But the most important rule:
    Once you've made your calculations -> Ground Test
     
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  8. Mar 25, 2020 at 3:10 PM #8

    ebruce1361

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    @Nytrunner Thanks for the clarification! You may remember me asking a similar question a couple of weeks ago about my scotch tube rocket, but ended up settling for scotch tape securing the payload bay to the main body. I'm still flying low and mid power and relying on the ejection charges that come built into the motors, but I'm planning on branching out to more advanced techniques here soon. Case in point, I recently drew up a simple CO2 release mechanism to compete with the Peregrine and Raptor devices from Tinder Rocketry. I have a pretty firm grasp of the how and why of CO2 deployment as well as BP deployment, but I still need to wrap my head around the relationships between the volume being pressurized and the pressures being released. Luckily, A Wee Dram is sufficiently powerful and sports a big enough payload bay that after I fly the three shots of scotch, I'll have a rocket that can carry a number of experimental payloads including deployment gizmos.
     
  9. Mar 25, 2020 at 4:18 PM #9

    Nytrunner

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    Hmmm, if you're using BP there are existing calculators.

    But if you're using CO2......I'm afraid you'll need to familiarize yourself with the Ideal Gas Law and wee dram of thermodynamics.
    Don't balk, it is manageable. It just sounds scary at first
     
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  10. Mar 25, 2020 at 5:01 PM #10

    ebruce1361

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    No worries here! I love diving into learning stuff like that!
     
  11. Mar 26, 2020 at 4:45 AM #11

    vcp

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    https://www.rocketryforum.com/threa...ruction-toolkit-software.155122/#post-1922582
     
  12. Mar 26, 2020 at 6:03 AM #12

    manixFan

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  13. Mar 28, 2020 at 5:39 AM #13

    bronicabill

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    Wouldn't happen to be Vince Heugle would it?
     
  14. Mar 28, 2020 at 12:33 PM #14

    Nytrunner

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    Nope, other Nasa guy, but that's a good guess
     
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